Induction heating apparatus using high frequency power of 10 kilohertz or more has been utilized in various fields and is particularly suitable for uniform heating of articles supplied continuously in large quantities to a heating zone of a heating coil. For example such apparatus has been used to heat the bottom surfaces of metal caps to cause resin sheets to adhere to the bottom surfaces. That is, when sealing a bottle by mounting a metal cap on the bottle mouth, a thermoplastic resin sheet or liner such as vinyl chloride, polyethylene or polypropylene is attached to the inner bottom surface of the cap and a bottle mouth is then sealed with the resin sheet forming a sealing material. In order to attach the resin sheet firmly to the inner bottom surface of the cap, an adhesive primer such as an epoxy is painted on the inner bottom surface of the cap and the resin sheet is then firmly attached with the primer in a hot melted state. Under ordinary circumstances, the adhesive primer will adhere the resin sheet firmly to the bottom surface of the cap when the bottom surface has been heated to about 100.degree. -200.degree. C., and it is in this heating process that high frequency induction heating apparatus is used. Bottle sealing caps so made include screw caps, crown caps and pilfer-proof caps, but in almost every case, regardless of cap type, the resin sheet is adhered to the inner bottom surface by means of primer in order that the sheet serves as the sealing material.
In order that a plurality of articles may be heated to a uniform temperature in an induction heating apparatus, it is necessary that a nearly uniform heating energy be applied to each article as it is supplied to the heating station of the apparatus and high frequency induction heating circuits that will satisfy this requirement have been previously sought after. However, prior heating circuits have been deficient in that the heating energy imparted to each article to be heated has varied depending on the number of objects supplied to the heating station. This is because the impedance and frequency of the heating coil circuits comprising current conductors of the prior circuits fluctuate markedly because of the mutual inductance with the articles to be heated. For example, in comparing a case when eighteen metal caps are supplied to a heating station to be heated simultaneously with the case when a single metal cap is supplied, there are marked variations in the load impedance and the frequency of the heating coils and the energy imparted to the articles under each of these cases fluctuates widely so that large fluctuations occur in heating temperature. In particular, in extreme cases when the quantity of metal caps being continuously supplied to the heating apparatus varies with time, as in metal cap production processes when the supply of caps to the heating station reaches zero, very wide differences in heating temperature occur because of variations of load impedance in the heating coil circuit and fluctuations in frequency.
The variations in the impedance and frequency properties of the heating coils differ depending on the type of article being heated, and in the case of non-magnetic articles such as aluminum, there is a tendency for the frequency to increase as the number of articles supplied to the heating coil increases. This is because the inductance component in the heating coil circuit decreases and the resistance component increases depending on the supply of the articles. When the heating coil circuit is generally under constant current control, an increase in the frequency will bring about an increase in the energy supplied to the articles to be heated, and as a result, when the number of objects supplied to the heating zone is decreased, insufficient heating will occur.
In the case of strongly magnetic articles such as caps made of tin plate or steel, there is a completely opposite tendency for the frequency and energy to decrease depending on the supply of articles to the heating coil. As a result when the number of articles comprising strongly magnetic materials is decreased in supply to the heating coil, the result will be one of overheating.
It is therefore an object of the present invention to provide for a high frequency induction heating apparatus having a circuit which will impart a nearly equal heating energy to each article to be heated even when the quantity of articles supplied to the apparatus fluctuates.